Apparatus and methods for heat loss pressure measurement

ABSTRACT

A heat loss gauge for measuring gas pressure in an environment includes a resistive sensing element and a resistive compensating element. The resistive compensating element is in circuit with the sensing element and is exposed to a substantially matching environment. An electrical source is connected to the sensing element and the compensating element for applying current through the elements. The current through the sensing element is substantially greater than the current through the compensating element. Measuring circuitry is connected to the sensing element and the compensating element for determining gas pressure in the environment to which the sensing element and compensating element are exposed based on electrical response of the sensing element and the compensating element.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/583,339, filed May 31, 2000, which is a continuation-in-partof U.S. application Ser. No. 09/475,392, filed Dec. 30, 1999, now U.S.Pat. No. 6,227,056, issued May 8, 2001, which is a Divisional of U.S.application Ser. No. 08/897,629, filed Jul. 21, 1997, now U.S. Pat. No.6,023,979, issued Feb. 15, 2000. The entire teachings of the aboveapplications and patents are incorporated herein by reference.

BACKGROUND

[0002] Because the rate of heat transfer through a gas is a function ofthe gas pressure, under certain conditions measurements of heat transferrates from a heated sensing element to the gas can, with appropriatecalibration, be used to determine the gas pressure. This principle isused in the well known Pirani gauge (shown in schematic form in FIGS. 1aand 1 b), in which heat loss is measured with a Wheatstone bridgenetwork which serves both to heat the sensing element and to measure itsresistance.

[0003] Referring to FIG. 1a, in a Pirani gauge the pressure sensorconsists of a temperature sensitive resistance RS connected as one armof a Wheatstone bridge. R2 is typically a temperature sensitiveresistance designed to have a negligible temperature rise due to thecurrent i₂. R3 and R4 are typically fixed resistences. RS and typicallyR2 are exposed to the vacuum environment whose pressure is to bemeasured. FIG. 1b illustrates an alternative bridge configuration.

[0004] Pirani gauges have been operated with constant current i₁ (asshown in U.S. Pat. No. 3,580,081), or with constant voltage across RS.In these methods, an electrical imbalance of the bridge is created whichreflects gas pressure. Pirani gauges have also been operated withconstant resistance RS (as shown in U.S. Pat. No. 2,938,387). In thismode, the rate at which energy is supplied is varied with changes in gaspressure, so the rate of change in energy supplied reflects changes ingas pressure. Each method of operation has differing advantages anddisadvantages, but the following discussion pertains particularly to theconstant resistance method and the configuration of FIG. 1a.

[0005] Voltage V_(B) is automatically controlled to maintain the voltagedifference between A and C in FIG. 1a at zero volts. When the potentialdrop from A to C is zero, the bridge is said to be balanced. At bridgebalance the following conditions exist:

i_(s)=i₂,  (1)

i₄=i₃,  (2)

i _(s) RS=i ₄ R 4,  (3)

and i ₂ R 2 =i ₃ R 3  (4)

[0006] Dividing Eq. 3 by Eq. 4 and using Eq. 1 and 2 gives

RS=βR 2  (5)

[0007] $\begin{matrix}{{{where}\quad \beta} = \frac{R4}{R3}} & (6)\end{matrix}$

[0008] Thus, at bridge balance RS is a constant fraction β of R2.

[0009] To achieve a steady state condition in RS at any given pressure,Eq. 7 must be satisfied:

Electrical power input to RS=Power radiated by RS+Power lost out ends ofRS+Power lost to gas by RS  (7)

[0010] A conventional Pirani gauge is calibrated against several knownpressures to determine a relationship between unknown pressure, P_(x),and the power loss to the gas or more conveniently to the bridgevoltage. Then, assuming end losses and radiation losses remain constant,the unknown pressure of the gas P_(x) may be directly determined by thepower lost to the gas or related to the bridge voltage at bridgebalance.

[0011] Because Pirani gauges may be designed to have wide range and arerelatively simple and inexpensive, there is a long-felt need to be ableto use these gauges as a substitute for much higher priced gauges suchas capacitance manometers and ionization gauges. However, existingdesigns leave much to be desired for accurate pressure measurement,especially at lower pressures.

[0012] Prior to 1977, the upper pressure limit of Pirani gauges wasapproximately 20 Torr due to the fact that at higher pressures thethermal conductivity of a gas becomes substantially independent ofpressure in macroscopic size devices. One of the present inventorshelped develop the CONVECTRON® Gauge produced and sold by the assignee(Granville-Phillips Company of Boulder Colorado) since 1977 whichutilizes convection cooling of the sensing element to provide enhancedsensitivity from 20 to 1,000 Torr. Hundreds of thousands of CONVECTRONOGauges have been sold worldwide. Recently several imitations haveappeared on the market.

[0013] Although the CONVECTRON® Gauge filled an unsatisfied need, it hasseveral disadvantages. It has by necessity large internal dimensions toprovide space for convection. Therefore, it is relatively large. Becauseconvection is gravity dependent, pressure measurements at higherpressures depend on the orientation of the sensor axis. Also, becausethe pressure range where gas conduction cooling is predominant does notneatly overlap the pressure range where convection cooling occurs, theCONVECTRON® Gauge has limited sensitivity from approximately 20 to 200Torr.

[0014] To help avoid these difficulties, microminiature Pirani sensorshave been developed which utilize sensor-to-wall spacings on the orderof a few microns rather than the much larger spacings, e.g., 0.5 in.,previously used. See for example U.S. Pat. Nos. 4,682,503 to Higashi etal. and 5,347,869 to Shie et al. W. J. Alvesteffer et al., in an articleappearing in J. Vac. Sci. Technol. A 13(6), November/December 1995,describe the most recent work on Pirani gauges known to the presentinventors. Using such small sensor to wall spacings provides a pressuredependent thermal conductivity even at pressures above atmosphericpressure. Thus, such microscopic sensors have good sensitivity from lowpressure to above atmospheric pressure and function in any orientation.

[0015] There are a number of problems with previous attempts to developmicrominiature gauges. Although microminiature sensors provide goodsensitivity over a large pressure range independent of orientation,their design is extremely complex and fabrication requires numerouselaborate processing steps in highly specialized equipment costinghundreds of thousands of dollars.

[0016] Microminiature sensors suffer from the same type of ambienttemperature-caused errors as do macroscopic sensors. All of the heatloss terms in Eq. 7 are dependent on ambient temperature and on sensingelement temperature at any given pressure. Thus, any attempt at pressuremeasurement with a Pirani gauge without temperature correction will beconfused by non-pressure dependent power losses caused by changes inambient temperature. All modern Pirani gauges attempt to correct for theerrors caused by ambient temperature changes. A widely used means forcorrecting for such errors is to use for R2 a temperature sensitivecompensating element RC in series with a fixed resistance R, as shown inFIGS. 1a and 1 b.

[0017] British Patent GB 2105047A discloses the provision of anadditional resistor to provide a potential divider. J. H. Leck, at page58 of Pressure Measurement in Vacuum, Chapman and Hall: London (1964)notes that Hale in 1911 made R2 of the same material and physicaldimensions as RS in his Pirani gauge. R2 was sealed off in its ownvacuum environment and placed in close proximity to RS. When thepressures at R2 and RS were equal, excellent temperature compensationwas achieved. However, at other pressures this means of temperaturecompensation is not very effective.

[0018] To avoid the extra cost and complexity of evacuating and sealingoff R2 in a separate bulb, R2 is conventionally placed in the samevacuum environment as RS. By making R2 with a relatively large thermalmass and large thermal losses, self heating of R2 can be madenegligible. Leck recommends that R2 be “made in two sections, forexample, one of copper and the other Nichrome wire . . . so that theoverall temperature coefficient (of R2) just matches that of the Piranielement itself (RS).” According to Leck, this method of temperaturecompensation has been used by Edwards High Vacuum of Great Britain inthe METROVAC® brand gauge. A similar temperature compensationarrangement is used in the CONVECTRON® brand gauge.

[0019] However, this technique (using two or more materials in R2 havingdifferent temperature coefficients of resistance to approximate thetemperature coefficient of RS) is effective only over a narrow range ofpressure. In fact the compensation can be made exact only at one, or atmost several temperatures as noted in U.S. Pat. No. 4,541,286, whichdiscloses this form of temperature compensation in a Pirani gauge. Also,the inventors have found that configurations with a large thermal masssignificantly increase the response time of the gauge to sudden changesin ambient temperature.

[0020] The inventors have also found, through extensive computersimulation, that using equal temperature coefficients for RS and R2 asrecommended by Leck and as practiced in the prior art does not providean entirely accurate temperature compensation. The inventors have alsofound that at pressures less than approximately 5×10⁻³ Torr, the endlosses exceed all other losses combined. The relative loss components asdetermined by this research (radiation loss, end loss and gas losscomponents of total loss) are shown in the graph of FIG. 2. At 1×10⁻⁵Torr, the end losses are over 1000 times greater than the gas loss andradiation losses are approximately 100 times greater than the gas loss.

[0021] Therefore, temperature change effects in prior art Pirani Gaugesare especially troublesome at very low pressures where gas conductionlosses are very low. Prior art heat loss gauges cannot measure very lowpressures accurately, for example, 1×10⁻⁵ Torr. The inventors havediscovered that this limitation is a result of failure to maintain endlosses in the sensing element sufficiently constant when ambienttemperature changes. The Alvesteffer-type Pirani gauge has thecapability of indicating pressure in the 10⁻⁵ Torr range, but does notprovide an accurate indication within that range. For example, if theend losses are not held constant to one part in 5,000 in a typicalPirani gauge, a pressure indication at 1×10⁻⁵ Torr may be off by 50% to100%.

[0022] The following analysis shows why prior art designs are ill-suitedto correct adequately for ambient temperature changes at low pressures.For convenience in examining the prior art, the problems are explainedusing examples of gauges with relatively large spacing of sensor elementto wall. It should be understood that the same type of problems exist inthe much more complex geometries of microminiature gauges, with sensingelement-to-wall spacings on the order of a few microns.

[0023]FIG. 3 is a schematic representation of a portion 302 of aconventional Pirani gauge using a small diameter wire sensing element304 and a compensating element 303. Those familiar with Pirani gaugedesign will appreciate that the components in FIG. 3 are not shown toscale, for ease of explanation and understanding. Typically, smalldiameter wire sensing element 304 is electrically and thermally joinedto much larger electrical connectors 306, 307 which are thermally joinedto much larger support structures 308, 309. Let T_(AL) represent thetemperature in support structure 308 at the left end of sensing element304 and T_(AR) represent the temperature in support structure 309 at theright end at any given time t. Let T_(SL) and T_(SR) represent thetemperatures at left sensing element connector 306 and right sensingelement connector 307 respectively. Let T_(CL) and T_(CR) represent thetemperatures at left compensating element connector 310 and rightcompensating element connector 311 respectively. Let T_(XL) and T_(XR)represent the temperatures a distance ΔX from connectors 306 and 307respectively. In prior art designs, it has apparently been assumed thatall of these temperatures are the same. However, the inventors havefound that even seemingly negligible differences assume great importancefor low pressure accuracy.

[0024] To better understand temperature compensation requirements, it isimportant to note several facts.

[0025] (1) At low pressures, the temperature of RC is determinedpredominantly by heat exchange between the compensating elementconnections and the compensating element. This is because at ambienttemperature and low pressures, radiation and gas conduction are veryinefficient means of exchanging heat from the compensating element toits surroundings relative to heat conduction through the ends of thecompensating element. Thus, at low pressures the compensating elementtemperature will be very close to the average of the temperatures of theconnectors at each end of the compensating element as shown in Eq. 8.$\begin{matrix}{T_{AVG} = \frac{T_{CL} + T_{CR}}{2}} & (8)\end{matrix}$

[0026] (2) The temperature of the electrically heated sensing elementvaries from the ends to center, increasing with distance from the coolersupports. Using finite element analysis the inventors have simulated thetemperature distribution along the sensing element. It has been foundthat with equal temperature coefficients of resistance for RS and RC,the temperature Tn of any segment n of the sensing element changes withchanges in average temperature T_(AVG) of the compensating element RC atconstant pressure at bridge balance so as to maintain a constantdifference ΔTn=Tn−T_(AVG). The difference ΔTn is a function of β and Rwhere R=R2−RC.

[0027] (3) According to Eq. 5, the sensing element resistance RS atbridge balance will be maintained at a resistance β times the resistanceelement R2. As the ambient temperature increases, the compensatingelement connectors also increase in temperature and thus the temperatureand resistance of RC will increase according to Eq. 8. Any increase inthe temperature and therefore the resistance of RC causes an increase inthe temperature and resistance of all segments of RS at bridge balance.

[0028] (4) The power losses out the ends of the sensing element dependon the temperature gradient γ at the ends of the sensing elementaccording to Eq. 9:

Power lost out end=k γ  (9)

[0029] where k is a constant and $\begin{matrix}{\gamma_{L} = {\frac{T_{XL} - T_{SL}}{\Delta \quad X}\quad {at}\quad {left}\quad {end}}} & (10) \\{\gamma_{R} = {\frac{T_{XR} - T_{SR}}{\Delta \quad X}\quad {at}\quad {right}\quad {{end}.}}} & (11)\end{matrix}$

[0030] If γ_(L) and or γ_(R) vary for any reason, then the end losseswill change and the pressure indication will be erroneous.

[0031] To understand in detail a significant deficiency in the prior artof temperature compensation at low pressures, assume that from a steadystate, T_(AR) is increased slightly for example by changes in the localambient temperature environment of the right support structure. AssumeT_(AL) remains unchanged. Because T_(AL) is assumed not to change,T_(CL) and T_(SL) will remain unchanged. However, the increase in T_(AR)will cause T_(CR) to increase by conduction of heat through theconnection. Thus, $T_{AVG} = \frac{T_{CL} + T_{CR}}{2}$

[0032] will increase. An increase in T_(AVG) will cause an increase inT_(XL) and T_(XR) at bridge balance, which will produce changes in γ_(L)and γ_(R). These changes in γ_(L) and γ_(R) will change the end lossterm in Eq. 7, causing an error in pressure measurement dependent on thesize of the changes in γ_(L) and γ_(R).

[0033] The inventors have determined that unless T_(AL) changes insubstantially the same way as T_(AR), sensing element end losses willnot remain unchanged whenever ambient temperature changes. Prior artPirani gauges have not been specifically designed to maintainT_(AL)=T_(AR) to the degree necessary for accurate low pressuremeasurement.

[0034] To understand another important deficiency in prior arttemperature compensation, assume that from a steady state, ambienttemperature is increased and that ambient temperature conditions aresuch that T_(AL)=T_(AR). Further assume the sensing element connectorsare of equal length but that the right compensating element connector issubstantially longer than the left compensating element connector as isthe case in a popular prior art Pirani gauge. Thus, T_(SL)=T_(SR) butT_(CR) will lag behind T_(CL) because of the assumed differences inlength. During this lag time when T_(CL)≠T_(CR), T_(AVG) will change,thus changing T_(XL) and T_(XR) at bridge balance. Thus, γ_(L) and γ_(R)will continually change during the lag time, producing errors in lowpressure indication.

[0035] The inventors have determined that unless the sensing element andcompensating element connectors have substantially identical physicaldimensions and substantially identical thermal properties, sensingelement end losses will not remain unchanged when ambient temperaturechanges. Prior art Pirani gauges have not been specifically designed sothat sensing and compensating element connectors have identical physicaldimensions and thermal properties.

[0036] Another significant deficiency arises (as the inventors havediscovered) from differences in mass between the compensating elementand the sensing element. Assume that the mass of the compensatingelement is substantially larger than that of the sensing element as istypically the case. With prior art Pirani gauges it is common practiceto make the compensating element large relative to the sensing elementand to provide a relatively large heat loss path to the compensatingelement surroundings so that the heat arising from dissipation ofelectrical power in RC can be readily dispersed. From a steady state,assume that ambient temperature increases and that T_(AL)=T_(AR) at alltimes. Thus, it will take a longer time for the compensating element toreach a new steady state temperature relative to the time it will takeT_(SL) and T_(SR) to reach a new steady state temperature. During thistime (which has been observed to be of several hours duration in apopular prior art Pirani gauge) T_(AVG) will continually change, thuscontinually changing T_(XL) and T_(XR) at bridge balance. Thus, γ_(L)and γ_(R) will change during the lag time, sensing element end losseswill not remain constant, and errors will be produced in low pressuremeasurement.

[0037] The same type of problems occur if the compensating element isdesigned to change temperature at a different rate than does the sensingelement with change in ambient temperature at bridge balance. Prior artdesigns such as the Alvesteffer-type device have this deficiency.

[0038] From their research, the inventors have determined that, unlessthe compensating element has been designed to change temperature at thesame rate as the sensing element, sensing element end losses continue tochange long after ambient temperature has stabilized at a new value.Yet, prior art Pirani gauges have not been designed to meet thisrequirement.

[0039] It has long been known to use for R2 a compensating element RC,with substantially the same temperature coefficient of resistance as thesensing element, in series with a temperature insensitive resistanceelement R so as to provide temperature compensation for gas losses andend losses which vary as the temperature difference between the sensingelement and its surroundings. This method of temperature compensationhas been employed in the CONVECTRON® Gauge for many years and is alsoused in the Alvesteffer gauge.

[0040] This method of temperature compensation assumes that if (1) thetemperature coefficients of resistance of the sensing and compensatingelements are equal; and (2) the change in sensing element resistance canbe made to rise in tandem with change in compensating elementresistance, then (3) the temperature of the sensing element will rise intandem with ambient temperature changes. Satisfying these twoassumptions is highly desirable, of course, because doing so wouldassure that the temperature difference between the heated sensingelement and the surrounding wall at ambient temperature would remainconstant as ambient temperature changes.

[0041] However, the inventors have found that prior art gauges whichutilize a constant resistance R in series with a temperature sensitiveresistance RC for R2 provide only partial temperature compensation aswill now be explained.

[0042] Assume that in FIG. 1a, R2 is composed of a temperature sensitivecompensating element RC and a temperature insensitive resistance R sothat

R 2=RC+R  (12)

[0043] Thus, Eq. 5 derived above for bridge balance may be written as

RS=β(RC+R)  (13)

[0044] where β is defined by Eq. 6 above.

[0045] Further, assume when the ambient temperature environment of thegauge is equal to T₁ that the sensing element operates at temperatureT_(S1) and the compensating element operates at temperature T_(C1). Thuswhen

T_(AMBIENT)=T₁  (14)

[0046] Eq. 13 may be written as

RS(T ₁)(1+α_(S)(T _(S1) −T ₁))=β[RC(T ₁)(1+α_(C)(T _(C1) −T ₁))+R]  (15)

[0047] Here, RS(T₁) is the resistance of the sensing element attemperature T₁, α_(S) is the temperature coefficient of resistance of RSat T₁, RC(T₁) is the resistance of the compensating element attemperature T₁, and α_(C) is the temperature coefficient of resistanceof R_(C) at T₁. Thus, when

T_(AMBIENT)=T₂

[0048] Eq. 13 may be written as

RS(T ₁)(1+α_(S)(T _(S2) −T ₁))=β[RC (T₁)(1+α _(C)(T _(C2) −T₁))+R]  (16)

[0049] Solving Eq. 15 for T_(S1) gives $\begin{matrix}{T_{S1} = {{{\left\lbrack {{\frac{\beta}{{RS}\left( T_{1} \right)}\left\lbrack {{{{RC}\left( T_{1} \right)}\left( {1 + {\alpha \quad {c\left( {T_{C1} - T_{1}} \right)}}} \right)} + R} \right\rbrack} - 1} \right\rbrack/\alpha}\quad s} + T_{1}}} & (17)\end{matrix}$

[0050] Solving Eq. 16 for T_(S2) gives $\begin{matrix}{T_{S2} = {{{\left\lbrack {{\frac{\beta}{{RS}\left( T_{1} \right)}\left\lbrack {{{{RC}\left( T_{1} \right)}\left( {1 + {\alpha \quad {c\left( {T_{C2} - T_{1}} \right)}}} \right)} + R} \right\rbrack} - 1} \right\rbrack/\alpha}\quad s} + T_{1}}} & (18)\end{matrix}$

[0051] Subtracting Eq. 17 from Eq. 18 gives the temperature change ΔT inthe sensing element RS when ambient temperature changes from T₁ to T₂.Thus, $\begin{matrix}{{\Delta \quad T} = {{T_{S2} - T_{S1}} = {{\beta \left( \frac{{RC}\left( T_{1} \right)}{{RS}\left( T_{1} \right)} \right)}\left( \frac{\alpha \quad c}{\alpha \quad s} \right)\left( {T_{C2} - T_{C1}} \right)}}} & (19)\end{matrix}$

[0052] Note that an effective compensating element is designed so thatits temperature closely follows ambient temperature. Thus, to a verygood approximation,

T _(C2) −T ₂ =T _(C1) −T ₁

[0053] or

T _(C2) −T _(C1) =T ₂ −T ₁  (20)

[0054] Thus, Eq. 19 may be written as $\begin{matrix}{{\Delta \quad T} = {{\beta \left( \frac{{RC}\left( T_{1} \right)}{{RS}\left( T_{1} \right)} \right)}\left( \frac{\alpha \quad c}{\alpha \quad s} \right)\left( {T_{2} - T_{1}} \right)}} & (21)\end{matrix}$

[0055] It is evident from Eq. 21 that the temperature change ΔT in thesensing element RS will be equal to the change in ambient temperatureT₂−T₁ only if $\begin{matrix}{{{\beta \left\lbrack \frac{{RC}\left( T_{1} \right)}{{RS}\left( T_{1} \right)} \right\rbrack}\left\lbrack \frac{\alpha \quad c}{\alpha \quad s} \right\rbrack} = 1} & (22)\end{matrix}$

[0056] Prior art gauges using a temperature sensitive compensatingelement RC in series with a fixed resistance R for R2 in FIG. 1a provideonly partial temperature compensation depending on the choice of β.Commercially available gauges having the design described by Alvestefferet al., the most recent work on Pirani gauges known to the presentinventors, would not satisfy Eq. 22.

[0057] As a third problem with prior art gauge designs, the inventorshave found that the level of power dissipation in R2 adversely affectsaccuracy. Prior art Pirani gauges, when configured as in FIG. 1a, havethe same pressure dependent current in RS as is in the compensatingelement at bridge balance. When configured as in FIG. 1b, at balance thesame pressure dependent voltage is applied across R2 as across RS. Ofcourse, a pressure dependent current in R2 will cause the temperature ofRC to rise above ambient temperature by an amount which varies withpressure.

[0058] Prior art Pirani gauges typically use a compensating element ofmuch larger physical dimensions than the sensing element, to dissipatethe heat and thus prevent excessive temperature in the compensatingelement. As noted above, different physical dimensions for the sensingand compensating elements cause measurement errors when ambienttemperature changes.

[0059] A fourth problem is that prior art Pirani gauges produce shiftsin pressure indications at low pressures when ambient temperaturechanges. Prior art Pirani gauges have used a variety of components inattempting to maintain the power lost by the sensing element unchangedas ambient temperature changes. For example, in U.S. Pat. No. 4,682,503thermoelectric cooling is used to control ambient temperature and thusminimize ambient temperature changes.

[0060] In the device disclosed in U.S. Pat. No. 4,541,286, a thermallysensitive element is mounted adjacent to the compensating arm of thebridge (actually glued to the exterior of the vacuum enclosure in acommercial version). Alvesteffer et al. use an additional element(designated therein as R4) in the bridge to help compensate for the factthat the temperature coefficient of resistance is slightly different forthe sensing element at operating temperature, compared to thecompensating element at ambient temperature. Although each of theseprior art hardware fixes remove some of the errors caused by changes inambient temperature, none of them removes substantially all of theerrors. Thus prior art Pirani gauges produce significant shifts inpressure indications at low pressures when ambient temperature changes.

[0061] Another prior system, disclosed in U.S. Pat. No. 5,608,168, linksvarious electrical measurements of the bridge (or approximationsthereof) and determines the value or temperature of the temperaturedependent resistance, and takes this parameter into account indetermining the pressure measurement. However, this system has increasedcomplexity because of the need to measure temperatures or other values.

[0062] Thus, there is a need for an improved Pirani-type gauge whichovercomes these problems.

SUMMARY OF THE INVENTION

[0063] The present invention provides improvements for heat losspressure measurement which cooperate synergistically to providesignificantly improved low, mid-range and high pressure measurementaccuracy, thus, permitting the range of accurate pressure measurement tobe extended to lower and to higher pressures within a single gauge.

[0064] As a first improvement, a small diameter wire sensing element ispositioned in the same plane as and spaced from a small diameter wirecompensating element with two parallel flat thermally conductive plates,each spaced 15 microns from the sensing and compensating elements. Inthis manner, the inventors have achieved high relative sensitivity insimple geometry without relying on convection. The extreme complexityand cost of microminiature Pirani gauge designs and the severaldisadvantages of convection cooling of the sensing element aresimultaneously avoided.

[0065] The inventors have found that this extremely simple, small, andinexpensive measuring means gives results up to atmospheric pressurecomparable to those obtained with very complex microminiature Piranigauges and to those obtained with much larger, position-sensitiveconvection-cooled Pirani gauges. Surprisingly, this improvement alsoprovides a sensing element with a volume of only 3% that of the sensingelement in the microminiature Alvesteffer gauge. The compensatingelement in the new device has a volume of less than 0.5% of theAlvesteffer-type compensating element.

[0066] The present invention also provides improved temperaturecorrection. The inventors have found that the accuracy of low pressuremeasurement can be significantly improved by better maintaining constantthe temperature gradient γ at the ends of the sensing element (see Eqs.10 and 11). The inventors have found that constancy of γ can be achievedby simultaneously:

[0067] 1. Using sensing and compensating elements with substantiallyidentical physical dimensions, thermal properties and resistanceproperties;

[0068] 2. Using sensing element and compensating element connectionswith substantially identical physical dimensions, thermal properties andresistance properties;

[0069] 3. Using element connections with substantially identical andlarge thermal conductances to a region of substantially uniformtemperature for all connections; and

[0070] 4. Locating the sensing and compensating elements in the samevacuum environment.

[0071] In the present invention, Eq. 22 is satisfied at all timesbecause the gauge is designed so that

RC(T _(A))=RS(T _(A))  (23)

[0072] where T_(A) is ambient temperature, and where

α_(C)=α_(S)  (24)

β=1  (25)

[0073] Another significant improvement is realized by providingnegligible heating in the compensating element. The inventors havemodified the conventional Wheatstone bridge to provide independentheating means for the sensing element, while producing substantiallyzero heating in any of the other three arms of the bridge. Thus, thecompensating element can be made with identical dimensions of thesensing element as well as identical physical properties. A DC heatingcurrent is used and confined to only the sensing element. A relativelysmall AC signal is used to sense bridge balance.

[0074] An additional performance improvement is realized by providing anew method of pressure compensation that results in accurate pressureindication at all pressures. In particular, the inventors havediscovered that an accurate indication of an unknown pressure P_(X) atbridge balance may be calculated from a simple equation of the form ofEq. 26.

P=f(VS,IS)  (26)

[0075] Where VS is the voltage drop across the sensing element and IS isthe current in the sensing element. The particulars of Eq. 26 arederived from paired values of VS_(C) and IS_(C) obtained by calibrationmethods for multiple known values of pressure P_(C) and ambienttemperature spread across the pressure and temperature ranges ofinterest, using three-dimensional curve fitting software. VS_(X) andIS_(X) are measured at the unknown pressure P_(X) at bridge balance andsubstituted into Eq. 26. Then, P_(X) is calculated using amicroprocessor or the like.

[0076] In this manner, the present invention provides significantadvancements in Pirani gauge accuracy, production cost, and packagesize.

[0077] The present invention further includes a heat loss gauge formeasuring gas pressure in an environment. The gauge includes a resistivesensing element and a resistive compensating element in circuit with thesensing element and being exposed to a substantially matchingenvironment. An electrical source is connected to the sensing elementand compensating element for applying current through the elements. Thecurrent through the sensing element is substantially greater than thecurrent through the compensating element. Measuring circuitry isconnected to the sensing element and the compensating element fordetermining gas pressure in the environment to which the sensing elementand compensating element are exposed based on electrical response of thesensing element and the compensating element.

[0078] In some embodiments, separate currents flow through the sensingelement and the compensating element. Current is applied to heat thesensing element to a temperature at which the resistance of the sensingelement matches the combined resistance of the compensating element plusthe value of a constant number of ohms. In embodiments where thecompensating element is positioned in series with anon-temperature-sensitive resistive element, current is applied to heatthe sensing element to a temperature at which the resistance of thesensing element matches the combined resistance of the compensatingelement and the non-temperature-sensitive resistive element. Gaspressure is determined based on heating current through the sensingelement and resulting voltage across the sensing element. In oneembodiment, separate DC currents flow through the sensing element andthe compensating element. The current through the compensating elementis a predetermined fraction of the current through the sensing elementso that the currents have a defined ratio. Feedback circuitry controlsthe levels of current through the sensing element and the compensatingelement. In another embodiment, a sensing current flows through both thesensing element and the compensating element, and a separate heatingcurrent flows through the sensing element.

[0079] In further embodiments, the sensing and compensating elements areof different lengths. In some embodiments, the compensating element isabout 5%-8% shorter in length, for 5%-8% lower resistance than thesensing element. In a specific embodiment, the compensating element is6%-7% shorter, about 6%-7% lower resistance, than the sensing element.In other embodiments, a parallel resistor is positioned across one ofthe sensing and compensating elements to trim the relative resistances.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0081]FIGS. 1a and 1 b are simplified schematic diagrams of conventionalPirani gauges;

[0082]FIG. 2 is a graph showing the components of heat loss in aconventional Pirani gauge, as discovered through the inventors'research;

[0083]FIG. 3 is a schematic representation of a conventional PiraniGauge using a small diameter wire for the sensing element;

[0084]FIG. 4a is a portion of an improved heat loss gauge according tothe present invention, and FIG. 4b is a cross-sectional view of theportion shown in FIG. 4a;

[0085]FIG. 5a is an enlarged cross-sectional view of the ends of animproved heat loss gauge according to the present invention, showingsupport and connection of sensing and compensating elements;

[0086]FIG. 5b is a cross-sectional view showing one embodiment of amechanism according to the present invention for maintaining spacingbetween heat conducting plates and a sensing element and compensatingelement, respectively;

[0087]FIG. 6 is a schematic diagram showing an independent heatingarrangement for a sensing element according to the present invention;

[0088]FIG. 7 is a schematic diagram showing another independent heatingarrangement for a sensing element according to the present invention;and

[0089]FIG. 8 is a schematic diagram showing yet another independentheating arrangement for a sensing element according to the presentinvention.

[0090]FIG. 9 is a schematic drawing of a portion of the circuit of FIG.7 having a trim resistor connected in parallel with the compensatingelement in an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0091] A description of preferred embodiments of the invention follows.The invention is first described in terms of four categories ofimprovements to conventional Pirani gauge designs. In a particularlypreferred embodiment, the four improvements are used together, andcombine synergistically to provide a Pirani gauge having substantiallyimproved performance characteristics. Additional embodiments of thepresent invention are then described thereafter.

[0092] Improvement 1

[0093] The first category of improvements will be discussed withreference to FIGS. 4a and 4 b. FIG. 4a is a side view of a portion 10 ofan improved heat loss gauge (not to scale). FIG. 4b is a sectional viewof portion 10 taken along line 4 b-4 b in FIG. 4a. As shown in FIGS. 4aand 4 b, a small diameter wire sensing element 12 is located in the sameplane and spaced a distance d from a small diameter wire compensatingelement 14. Spacing d between sensing element 12 and compensatingelement 14 is preferably approximately 0.030 in. but may range from0.010 in. to 0.200 in. Parallel plates 16 and 16′ are provided proximateto and parallel to sensing element 12 and compensating element 14.

[0094] Parallel plates 16 and 16′ are positioned a distance S fromsensing element 12 and compensating element 14. S is preferably 0.0007in. but may range from 0.0002 in. to 0.002 in. Sensing element 12 ismade of a material with a high temperature coefficient of resistance,such as pure tungsten, which may be gold plated to help assure aconstant emissivity.

[0095] The diameter of sensing element 12 is preferably 0.0005 in. butmay range from 0.0001 in. to 0.002 in. Although a cylindrical wire shapeis preferred, other shapes such as a ribbon may be used for both thesensing and compensating elements. The length of sensing element 12 ispreferably 1 in. but may range from 0.25 in. to 3 in.

[0096] Compensating element 14 is made of the same material as thesensing element 12 with the same physical dimensions, and with the samethermal and resistance properties.

[0097] Portion 10 of the heat loss gauge may be installed in a measuringcircuit of the type shown in FIG. 6, in a manner which will be describedin more detail below.

[0098] Parallel plates 16 and 16′ conduct heat and thereby tend toequalize—temperature gradients along heated sensing element 12 andbetween the ends of sensing element 12 and compensating element 14. Inthis manner, the invention achieves high relative sensitivity with asimple structure, and without relying on convection. In this embodimentof the invention, the accuracy of low pressure measurement issignificantly improved by using sensing and compensating elements withsubstantially identical physical dimensions, thermal properties andresistance properties, and locating the sensing and compensatingelements in the same vacuum environment. Using this design, the extremecomplexity and cost of microminiature Pirani gauge designs anddisadvantages associated with convection cooling of the sensing elementare simultaneously avoided. This improvement permits pressuremeasurement results up to atmospheric pressure comparable to thoseobtained with very complex microminiature Pirani gauges, and comparableto those obtained with much larger, position sensitive convection cooledPirani gauges.

[0099] Improvement 2

[0100] As a second broad feature of the invention, an improved mountingarrangement is provided for the sensing and compensating elements. Theaccuracy of low pressure measurement is significantly improved by usingsensing element and compensating element connections with substantiallyidentical physical dimensions, thermal properties and resistanceproperties, and by using element connections with substantiallyidentical and large thermal conductances to a region of substantiallyuniform temperature for all connections.

[0101]FIG. 5a is a greatly enlarged cross-sectional view of one end ofgauge portion 10 where the sensing element 12 is supported by andelectrically connected to sensing element connectors 20 and 20′ and thecompensating element 14 is shown supported by and electrically connectedto compensating element connectors 22 and 22′. The section of FIG. 5a istaken along line 5 a-5 a in FIG. 4a. Preferably, identical supports (asshown in FIG. 5a) are provided at each end of gauge portion 10.

[0102] Connectors 20, 20′, 22 and 22′ are preferably made of platinumribbon, 0.001 in. thick by 0.060 in. wide. Plates 16 and 16′ arepreferably made of an electrically insulating material with a highthermal conductivity such as aluminum nitride.

[0103] Alternatively, sensing and compensating element connectors 20,20′, 22 and 22′ can be electrically insulated from the plates 16 by thinelectrically insulating layers 24 and 24′ which may be a diamond-likecoating on tungsten. In this case, Plates 16 and 16′ may be made of ahigh thermal conductivity material such as tungsten. Preferably, theselected material has a thermal conductivity greater than 0.25watts/cm/K.

[0104] Plates 16 and 16′ are held in position by simple sheet metalclamps at each end (not shown). The clamps apply sufficient force to theplates 16 and 16′ to embed the sensing element 12 and the compensatingelement 14 into the connectors 20, 20′, 22 and 22′ until the connectors20 and 20′, and 22 and 22′ are in intimate contact. Thus, the spacing Sbetween the sensing element 12 and the surface of the plates 16 and 16′is determined by the diameter of the sensing element and the thicknessof the thin ribbon connectors 20, 20′, 22, and 22′. This feature of thepresent invention permits a sensing element smaller than a human hair tobe spaced a comparable distance from two flat surfaces, precisely andvery inexpensively as well as providing electrical connections toadditional circuitry.

[0105] Plates 16 and 16′ provide a region of substantially uniformtemperature, especially when isolated in vacuum with minimal thermalconductivity to the outside world. The thin ribbon connectors 20, 20′,22 and 22′ provide identical dimensions, short path and very largethermal conductances to said region of uniform temperature, thussatisfying several of the conditions for constancy of temperaturegradient, γ, at the ends of the sensing element.

[0106] Sensing element 12 may be suitably tensioned as shown in FIG. 5bby a small diameter wire spring 26 which is loaded during assembly andbears on sensing element 12 adjacent to said connector 21 of sensingelement 12. Spring 28 is used in a similar manner to tension thecompensating element 14. Springs 26 and 28 serve to maintain precisespacing of the sensing element 12 and compensating element 14 relativeto plates 16 and 16′ as ambient temperature changes. Sufficient slackmust be built into the sensing element 12 and compensating element 14assemblies to prevent breakage due to differential thermal expansion ofthe elements 12 and 14 and the plates 16. Without the springs 26 and 28,this slack would change with ambient temperature, thus preventingmaintenance of constant spacing S between the parallel plates 16 and 16′and the sensing and compensating elements, respectively, and causingmeasurement errors.

[0107] In the design according to this embodiment of the invention, Eq.22 is partially satisfied by the fact that sensing element 12 andcompensating element 14 are physically, electrically, and thermallyidentical. In addition, R3 is set equal to R4 in the embodiment of FIG.6, which from Eq. 6 assures that β=1. Thus, Eq. 22 is fully satisfied atall times by this design.

[0108] Improvement 3

[0109] A third major feature of the invention is an apparatus and methodfor independently heating sensing element 12. This improvement isillustrated in FIG. 6 wherein a Wheatstone bridge 30 is modified toprovide independent heating of sensing element 12. Prior art circuits,used with a compensating element with the same physical dimensions andmade of the same material as the sensing element as in the presentinvention, cause the compensating element to operate not at ambienttemperature but at the same temperature as the sensing element. Thus,Pirani gauges with the inventive improvements described above cannotachieve their accuracy potential using prior art heating circuits.

[0110] Referring now to FIG. 6, a Wheatstone bridge 30 with nodes A, B,C, and D is provided with sensing element 12 having resistance value RS,connected between nodes B and C. Non-temperature sensitive resistanceelement 15 (having resistance R) and compensating element 14 (havingresistance RC) together make up resistance R2. R2 and capacitor 36 areconnected in series order between nodes C and D. Resistor 17 havingvalue R4 is connected between nodes A and B, and resistor 19 havingvalue R3 is connected between nodes A and D. Vacuum environment 34encloses sensing element 12 and compensating element 14. AC voltagesource 38 is connected between nodes B and D, and frequency selectivedetector 40 is connected between nodes A and C. DC current source 32 isconnected between nodes B and C to provide current to node B. Controller42 is connected, via automatic feedback linkages 46 and 47, so as tocontrol DC current source 32 and so as to receive a voltage detectioninput from frequency selective detector 40 for purposes of that control.

[0111] Vacuum environment 34 encloses a portion 10 (as shown in FIGS. 4aand 4 b and described above with reference to those Figures) comprisingsensing element 12, compensating element 14, and plates 16 and 16′. Inaddition, the assembly method described previously with reference toFIGS. 5a and 5 b is preferably used in the circuit of FIG. 6. Elementconnectors 20 and 20′ at one end of sensing element 12 (shown in FIG.5a) are electrically connected to Point C in bridge circuit 30 of FIG.6, while sensing element connectors 21 and 21 (not shown) at the otherend of sensing element 12 are electrically connected to Point B in FIG.6. Compensating element connectors 22 and 22′ at one end of compensatingelement 14 (shown in FIG. 5a) are electrically connected throughcapacitor 36 to Point D in FIG. 6, while the other end of compensatingelement 14 is connected to compensating element connectors 23 which areconnected through a resistance 15 to Point C.

[0112] As shown in FIG. 6, DC current source 32 furnishes heatingcurrent I to sensing element 12 which is located in the vacuumenvironment 34. A capacitor 36 is provided as a means for preventingcurrent from current source 32 from being present in R2, R3 and R4.Thus, unlike prior art Pirani gauges using a conventional Wheatstonebridge, no portion of the heating current or heating voltage in RS ispresent in R2 at any time.

[0113] AC voltage source 38 applies an AC signal voltage to bridge 30producing AC signal currents i_(S), i₂, i₃, and i₄. Using very smallvalues for i_(S), i₂, i₃, and i₄ and frequency selective detector 40,bridge balance can be detected with negligible heating produced in anyarm of bridge 30. The DC current I from source 32 is automaticallyadjusted by controller 42, so as to continually assure that the ACvoltage drop i_(S)RS from point B to C is equal to the voltage drop i₄R4from B to A as measured by the AC voltage detecting function offrequency selective detector 40. This automatic feedback linkage isindicated by dashed lines 46 and 47.

[0114] Processor 51 is connected to current meter 49 and to voltagemeter 48, and produces an output indicative of pressure in the vacuumenvironment 34 based on the level of heating current and through sensingelement 12 and the voltage drop across sensing element 12.

[0115] Thus, compensating element 14 may be made with the same physicaldimensions and thermal and resistance properties as sensing element 12and still operate at ambient temperature without any pressure dependentelectrical heating.

[0116] Improvement 4

[0117] A fourth improvement will be described with reference again toFIG. 6. In this improvement, an improved apparatus and method areprovided for calibrating and operating the Pirani gauge according to thepresent invention.

[0118] The inventors have discovered that an accurate indication of anunknown pressure P_(X) at bridge balance may be calculated from a simpleequation of the form of Eq. 26.

P=f(VS,IS)  (26)

[0119] This finding differs from more conventional approaches. Pressureindication has been considered to depend not only on resistance of thesensing element, but also on other factors such as ambient temperature.Thus, conventional calibration schemes often require measurements ofresistance and other quantities, both for calibration and duringoperation. However, the inventors have discovered that when theimprovements described above are made, the values of VS and ISincorporate sufficient temperature information to produce an accuratepressure output, so that it is possible to eliminate the steps ofseparately measuring other parameters such as ambient temperature. Inthis manner, it is possible to use a three dimensional calibration tableto determine pressure based on voltage and current alone.

[0120] In order to calibrate the gauge shown in FIG. 6, sensing element12 is exposed to a series of known representative pressures and ambienttemperatures spread over the pressure and temperature ranges ofinterest. The voltage drop, VS_(C), as measured by voltmeter 48 and thecurrent, IS_(C), as measured by current meter 49 are recorded togetherat bridge balance with each of the known representative calibrationpressures, P_(C). These values may be recorded by a program operating inprocessor 51 or may be transferred to another processing unit forcalibration calculations. The pressure P_(C) is plotted against voltageVS_(C) and current IS_(C). Each series of measurements at a givencalibration temperature produces a constant temperature functionrelating pressure to voltage and current. Significantly, as noted above,the inventors have discovered that these constant temperature functionscan be usefully combined in a single three-dimensional data table todefine a single calibration function of the form of Eq. 26. When this isdone, the result is a series of points defining a surface, where theheight of the surface is the pressure and is a function of measuredvoltage and current values.

[0121] The resulting calibration data may be stored in a lookup tableand measured pressures can be determined by interpolating betweenpressure values stored in the lookup table based on the measured voltagedrop and current. However, because of the number of points that must bestored to produce accurate output over a wide range of pressures, in thepreferred embodiment, an approximating equation is obtained for thesurface on which the measured values lie. This can readily beaccomplished using three-dimensional surface plotting software. Theresulting equation is of the form shown in Eq. 26. Then, to measure anunknown pressure P_(X) at any temperature, VS_(X) is measured byvoltmeter 48 and IS_(X) is measured by current meter 49 at bridgebalance. The correct value of pressure can then be readily obtained bysubstitution in Eq. 26 giving

P _(X) =f(VS _(X) ,IS _(X))  (27)

[0122] For convenience, Eq. 27 can be stored in processor 51 which canthen be used to automatically calculate P_(X) when VS_(X) and IS_(X) areinput to processor 51.

[0123] Those skilled in the art will appreciate that other quantitiescould be substituted for voltage and current within the scope of thisinvention. For example, a function of the form P_(X)=g (W, R) where W ispower applied to sensing element 12 and R is the resistance of sensingelement 12 could be used in place of Equation 27. In this case, W and Rcan be calculated from the output of voltmeter 48 and current meter 49.What is important is that the two selected parameters includeinformation relating to both current and voltage, such that the effectsof changes in current and voltage will be differentially reflected inthe calibration graph or table created based on values of the twoparameters. Thus, for example, the two input parameters for the functionmay be any two of a group including: power, current, voltage, andresistance. To generalize, it is possible to identify an equation of theform

P=h(X,Y)

[0124] which approximates the calibration surface, where X is the firstinput parameter, Y is the second input parameter, and P is the pressurecorresponding to values of the first parameter X and second parameter Y.This equation is then used as a proxy for the multi-dimensionalcalibration surface to calculate the pressure.

[0125] This improvement provides excellent temperature compensation from0° C. to 50° C. from pressures less than 10⁻⁴ Torr to above atmosphericpressure. It avoids the need to measure power and temperature as issometimes done. It compensates for all types of ambient temperaturechange induced errors, such as change in radiation loss, not merelythose losses dependent on changes in sensing element to wall temperaturechanges as is the case in U.S. Pat. No. 4,682,503. The improvementavoids the complexity of having to control the ambient temperature usingthermoelectric cooling as described in U.S. Pat. No. 5,347,869. Inaddition, this improved calibration and operating method automaticallycompensates for the fact that the temperature coefficient of resistivitywill be slightly different for the sensing element at operatingtemperature than for the compensating element at ambient temperature.

[0126] Further Embodiments

[0127] Referring to FIG. 7, gauge 60 is an embodiment of a gauge thatdiffers from that depicted in FIG. 6 in that it does not employ aWheatstone bridge. As in FIG. 6, gauge 60 includes a sensing element 12(having a resistance value RS), a non-temperature-sensitive resistanceelement 15 (having a resistance R) and a temperature compensatingelement 14 (having a resistance RC), with elements 12 and 14 beingpositioned within a vacuum environment 34 in a similar manner orarrangement. Although the circuitry that connects elements 12, 14 and 15in gauge 60 differs from that in FIG. 6, elements 12, 14 and 15 areutilized in a similar manner as in FIG. 6. For example, sensing element12 is heated while non-temperature-sensitive resistance element 15 andtemperature compensating element 14 are not heated significantly. Inaddition, the voltage VS across and the current IS through sensingelement 12 are measured and utilized to determine pressure in a similarmanner.

[0128] Gauge 60 includes a power source 61 for supplying power tocurrent sources 62 and 64 via lines 74 and 76, respectively. Currentsources 62 and 64 are interdependent and preferably provide DC currentto element 12 and elements 14/15, respectively. Current source 64provides a current of a magnitude or level that is a predeterminedfraction of that provided by current source 62. In the example shown inFIG. 7, current source 64 provides {fraction (1/10)} the amount ofcurrent provided by current source 62.

[0129] The current IS provided by current source 62 is directed throughsensing element 12 by way of line 78, node 80 and line 88. Thefractional current provided by current source 64 is directed throughnon-temperature-sensitive resistance element 15 and temperaturecompensating element 14 by way of line 100, node 102 and line 103(positioned between elements 14 and 15). As in FIG. 6, elements 14 and15 compensate for ambient temperature changes. Directing a fraction ofthe sensing current IS through the temperature compensating element 14makes the temperature rise of the compensating element 14 insignificantrelative to that of sensing element 12. In the example depicted in FIG.7 where the ratio of the sensing current IS to the fractional currentthrough temperature compensating element 14 is a ratio of 10:1, thepower dissipated in the temperature compensating element 14 is less than{fraction (1/100)} the power dissipated in sensing element 12 due to thesquared relationship of current to power (where Power=I²R). As a result,the temperature rise of compensating element 14 is less than 1%, thanthat of sensing element 12. Although a current ratio of 10:1 for currentsources 62 and 64 is described, other ratios less than or greater than10:1 may be employed. The current IS is controlled to heat the sensingelement 12 to a temperature level at which the resistance RS increasesto equal the combined resistance R+RC. It is at that temperature thatcalibration data defines the environmental pressure. A feedback circuitmaintains the current at that level.

[0130] Specifically, the voltage V₂ across the combined resistance R+RCindicative of the resistance R+RC, is applied through a unity multiplier66 (connected to nodes 102 and 98) to a summing circuit 70 while thevoltage V₁ across the resistance RS, indicative of the resistance RS, isapplied through a multiplier 68 (connected to nodes 84 and 92) to thesumming circuit 70. Since R=V/I, and the current through R+RC is onetenth the current through RS, the voltage across R+RC is one tenth thevoltage across RS when the resistences are equal. Therefore, to make acomparison of the voltages to determine whether the resistances areequal, the voltage across RS must be multiplied by one tenth of themultiplier applied to the voltage across R+RC. The multiplier 68multiplies the voltage V₁ by −.1 to assure that the multipliers have aninverse ratio to the ratios of the current IS and IS/10. The negativemultiplier 68 allows the summer circuit 70 to perform a subtraction ofthe normalized resistance voltages to provide an error signal 110indicating a difference between the resistances. That difference isamplified in high-gain integrating error amplifier 72 and fed back tocontrol the current level IS. The output of the error-amplifier 72 isfed back in parallel to current sources 62 and 64 via line or feedbackloop 112, node 114 and lines 116 and 118 to adjust the level of currentprovided by current sources 62/64 as necessary. The currents provided bycurrent sources 62/64 are adjusted such that the resistences RS ofsensing element 12 and R+RC of elements 15/14 maintain a predeterminedequilibrium. In the example shown in FIG. 7, the resistances are matchedsuch that RS=R+RC. Alternatively, other ratios or resistance levels maybe employed where RS is less than or greater than R+RC. In addition,although a feed back circuit for controlling current sources 62/64 ispreferred, alternatively, such a feedback circuit may be omitted withthe database allowing for different voltage ratios. When the feed backcircuit is omitted, the response time of the gauge 60 is typicallyslower.

[0131] The current through sensing element 12 is determined by currentsensor 49, and the voltage V₁ across sensing element 12 due to thatcurrent is determined by sensor 48, which is connected to nodes 80 and92 with lines 82 and 86. Elements 12 and 14 are connected to nodes 92and 98 by lines 90 and 105, respectively, and line 86 is connected toground by node 94. Sensor 48 and multipliers 66 and 68 have high inputimpedences so all current IS flows through sensing element 12. As in theprior embodiment, current and voltage parameters can be used in a database lookup to determine the pressure to which elements 12 and 14 areexposed.

[0132] Gauge 60 is calibrated in a similar manner as that described forthe gauge depicted in FIG. 6 where sensing element 12 is exposed to aseries of known representative pressures and ambient temperatures spreadover the pressure and temperature ranges of interest. The voltage dropacross sensing element 12 is measured by voltmeter 48 and the currenttherethrough is measured by current meter 49 while resistances RS andR+RC are maintained at a predetermined equilibrium for example, RS=R+RC.These values are plotted to provide the three-dimensional data table andsurface described in regard to FIG. 6.

[0133] As a result, in use, an unknown pressure can be determined bymeasuring voltage and current across sensing element 12 and thenemploying the methods described for the gauge of FIG. 6. For example,the values of voltage and current that are measured are compared to thestored calibration data which contains pressure values for particularvoltage and current values. Typically, the measured values of voltageand current do not exactly match any of the values contained within thestored data. Consequently, the value of the measured pressure isdetermined by interpolation of the stored voltage/current/pressurecalibration data. Preferably, an approximating equation such as Eq. 27is employed to perform the interpolation. By storing Eq. 27 in theprocessor, pressure may be automatically calculated from the measuredvoltage and current across sensing element 12. As in the gauge of FIG.6, quantities other than voltage and current may be employed fordetermining pressure within the scope of the invention.

[0134] Referring to FIG. 8, gauge 125 is an embodiment of a gauge inaccordance with the present invention that differs from gauge 60 (FIG.7) in that the positive (+) input of multiplier 66 is moved from node102 to a new node 120 located between elements 14 and 15 as shown. Inaddition, a third multiplier 121 with gain K is included. Multiplier 121is connected to node 102 and a new node 119 (located between elements 14and 15) as shown. In this arrangement, the resistance of line 103between elements 14 and 15 does not add an uncertain incremental valueto the resistence R of element 15. As a result, line 103 can be alengthy wire if required, and the resistance between nodes 119 and 120remains substantially inconsequential to the accuracy of gauge 125. Thisallows element 15 to be positioned in more convenient locations such ason a printed circuit board of an electronics package rather than at thetransducer and the resistance of the termination of the compensatingelement 14 need not be so tightly controlled. Furthermore, positioningelement 15 away from the transducer allows element 15 to be in anenvironment that has a more stable temperature so that unintentionaltemperature sensitivities are minimized. The value of the resistance Rof element 15 can be selected based on cost, convenience, oravailability since the voltage drop across element 15 can be multipliedby any arbitrary value K by multiplier 121 to obtain the desiredresults.

[0135] Still further embodiments of the present invention can beconfigured such that the sensing element 12 and the temperaturecompensating element 14 are different in resistance as well as length.The sensing element 12 and the temperature compensating element 14 canbe configured such that the resistance of the temperature compensatingelement 14 is slightly less than the sensing element 12, for example, upto about 10% less. It has been found that in some situations, acompensating element 14 which has slightly less resistance than thesensing element 12 can provide further improved ambient temperaturecompensation of the pressure signal over a compensating element 14having a resistance matching the sensing element 12.

[0136] In one embodiment, where the sensing 12 and compensating 14elements have equal lengths and resistances, the resistance of thecompensating element 14 can be lowered to the desired amount by a trimresistor 126 that is electrically connected in parallel with thecompensating element 14, and generally positioned externally to thesensed or measured environment. FIG. 9 depicts an example of a trimresistor 126 connected in parallel with the compensating element 14shown in the circuit of FIG. 7, where parallel trim resistor 126 isconnected to line 103 via line 124, and node 122, and to line 105 vialine 128 and node 130. When the parallel trim resistor 126 is inparallel with compensating element 14, the resistor 126 becomes part ofcompensating element 14. In one embodiment, compensating element 14 isprovided with about 4.5% lower resistance than sensing element 12 forproviding temperature compensation optimized at about 300 mTorr. Theaddition of the parallel trim resistor 126 in parallel with thecompensating element 14 changes both the temperature coefficient ofresistance and the ohms/° C. of the circuit leg of the compensatingelement 14.

[0137] In another embodiment, the compensating element 14 is made of thesame material and is generally the same diameter as the sensing element12 but is slightly shorter in length in order to provide the lowerresistance relative to the sensing element 12. The length difference isdetermined empirically by comparison over a temperature range with acapacitance diaphragm gauge. The length mismatch difference is smallenough so that the sensing 12 and compensating 14 elements still havesimilar temperature response and physical characteristics, but theshorter compensating element 14 provides better temperaturecompensation. In one embodiment, the compensating element is about 6.4%shorter (about 6.4% less resistance) than sensing element 12, whichtemperature compensates the sensor voltage curve optimized at about 1Torr. A shorter compensating element 14 has a temperature coefficient ofresistance (ohms/ohm/° C.) that is the same as the sensing element 12but the ohms/° C. is different.

[0138] The optimum length and resistance mismatch between the sensingelement 12 and the compensating element 14 can vary between differentgauges depending upon the length, position, and spacing of the sensing12 and compensating 14 elements, as well as the proximity to othercomponents. The length mismatch can be employed in any of the circuitspreviously described.

[0139] A parallel trim resistor 126 can be employed in situations forlowering the resistance of compensating element 14 where the length orresistance of compensating element 14 is less than the sensing element12 but not to the full extent desired. In addition, if the length orresistance of compensating element 14 relative to sensing element 12 islower than desired, a parallel trim resistor 126 can instead be placedin parallel with sensing element 12 to reduce the resistance of thesensing element 12 so that the resistance of compensating element 14 isincreased relative to the resistance of sensing element 12 to bring therelative resistances into the desired range. In such a case, theparallel trim resistor 126 becomes part of the sensing element 12.Although the parallel trim resistor 126 has been described above asbeing employed in the circuit of FIG. 7, it is understood that theparallel trim resistor 126 can be employed in any of the other circuitsdescribed above.

[0140] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

[0141] For example, features of the different embodiments of the presentinvention may be substituted for each other or combined. In addition,although gauges 60 and 125 have been described to apply DC current toelements 12, 14 and 15, other embodiments of the present inventioninclude applying AC current to elements 12, 14 and 15. Furthermore,although specific dimensions and specifications have been provided, itis understood that the dimensions and specifications can be varieddepending upon the situation at hand.

What is claimed is:
 1. A heat loss gauge for measuring gas pressure inan environment comprising: a resistive sensing element; a resistivecompensating element in circuit with the sensing element and beingexposed to a substantially matching environment; at least one electricalsource connected to the sensing element and compensating element forapplying current through the elements, the current through the sensingelement being substantially greater than the current through thecompensating element; and measuring circuitry connected to the sensingelement and the compensating element for determining gas pressure in theenvironment to which the sensing element and compensating element areexposed based on electrical response of the sensing element and thecompensating element.
 2. The gauge of claim 1 further comprisingfeedback circuitry for controlling the current through the sensingelement and the compensating element.
 3. The gauge of claim 2 whereinthe compensating element is in series with a non-temperature-sensitiveresistive element.
 4. The gauge of claim 3 wherein the electrical sourceapplies current to heat the sensing element to a temperature at whichthe resistance of the sensing element matches the combined resistance ofthe compensating element and the non-temperature-sensitive resistiveelement.
 5. The gauge of claim 1 wherein the electrical source appliescurrent to heat the sensing element to a temperature at which theresistance of the sensing element matches the combined resistance of thecompensating element plus a constant number of ohms.
 6. A heat lossgauge for measuring gas pressure in an environment comprising: aresistive sensing element; a resistive compensating element in circuitwith the sensing element and being exposed to a substantially matchingenvironment; electrical means connected to the sensing element andcompensating element for applying current through the elements, thecurrent through the sensing element being substantially greater than thecurrent through the compensating element; and measuring means connectedto the sensing element and the compensating element for determining gaspressure in the environment to which the sensing element andcompensating element are exposed based on electrical response of thesensing element and the compensating element.
 7. A heat loss gauge formeasuring gas pressure in an environment comprising: a resistive sensingelement; a resistive compensating element in circuit with the sensingelement and being exposed to a substantially matching environment; anelectrical source connected to the sensing element and compensatingelement for applying current through the elements, the current throughthe sensing element being substantially greater than the current throughthe compensating element, the sensing and compensating elements being ofdifferent lengths; and measuring circuitry connected to the sensingelement and the compensating element for determining gas pressure in theenvironment to which the sensing element and compensating element areexposed based on electrical response of the sensing element and thecompensating element.
 8. The gauge of claim 7 in which the compensatingelement is shorter than the sensing element.
 9. The gauge of claim 8 inwhich the sensing and compensating elements are configured to providethe compensating element with about 5%-8% lower resistance than thesensing element.
 10. The gauge of claim 9 in which the sensing andcompensating elements are configured to provide the compensating elementwith about 6%-7% lower resistance than the sensing element.
 11. Thegauge of claim 8 in which the compensating element is about 5%-8%shorter in length than the sensing element.
 12. The gauge of claim 11 inwhich the compensating element is about 6%-7% shorter than the sensingelement.
 13. The gauge of claim 7 in which a parallel resistor ispositioned across one of the sensing and compensating elements forproviding the compensating element with lower resistance.
 14. A heatloss gauge for measuring gas pressure in an environment comprising: aresistive sensing element; a resistive compensating element in circuitwith the sensing element and being exposed to a substantially matchingenvironment; electrical means connected to the sensing element andcompensating element for applying current through the elements, thecurrent through the sensing element being substantially greater than thecurrent through the compensating element, the sensing and compensatingelements being of different lengths; and measuring means connected tothe sensing element and the compensating element for determining gaspressure in the environment to which the sensing element andcompensating element are exposed based on electrical response of thesensing element and the compensating element.
 15. A method of measuringgas pressure in an environment comprising: providing a resistive sensingelement; providing a resistive compensating element that is in circuitwith the sensing element and is exposed to a substantially matchingenvironment; applying current through the sensing element andcompensating element from at least one electrical source, with thecurrent through the sensing element being substantially greater than thecurrent through the compensating element; and with measuring circuitryconnected to the sensing element and the compensating element,determining gas pressure in the environment to which the sensing elementand compensating element are exposed based on electrical response of thesensing element and the compensating element.
 16. The method of claim 15further comprising controlling the current through the sensing elementand the compensating element with feedback circuitry.
 17. The method ofclaim 16 further comprising positioning the compensating element inseries with a non-temperature-sensitive resistive element.
 18. Themethod of claim 17 further comprising applying current from theelectrical source to heat the sensing element to a temperature at whichthe resistance of the sensing element matches the combined resistance ofthe compensating element and the non-temperature-sensitive resistiveelement.
 19. The method of claim 15 further comprising applying currentfrom the electrical source to heat the sensing element to a temperatureat which the resistance of the sensing element matches the combinedresistance of the compensating element plus a constant number of ohms.20. A method of measuring gas pressure in an environment comprising:providing a resistive sensing element; providing a resistivecompensating element that is in circuit with the sensing element and isexposed to a substantially matching environment; applying currentthrough the sensing element and compensating element from an electricalsource, with the current through the sensing element being substantiallygreater than the current through the compensating element; configuringthe sensing and compensating elements to be of different lengths; andwith measuring circuitry connected to the sensing element and thecompensating element, determining gas pressure in the environment towhich the sensing element and compensating element are exposed based onelectrical response of the sensing element and the compensating element.21. The method of claim 20 further comprising configuring thecompensating element to be shorter than the sensing element.
 22. Themethod of claim 21 further comprising configuring the sensing andcompensating elements to provide the compensating element with about5%-8% lower resistance than the sensing element.
 23. The method of claim22 further comprising configuring the sensing and compensating elementsto provide the compensating element with about 6%-7% lower resistancethan the sensing element.
 24. The method of claim 21 further comprisingproviding the compensating element with a length that is about 5%-8%shorter than the sensing element.
 25. The method of claim 24 furthercomprising providing the compensating element with a length that isabout 6%-7% shorter than the sensing element.
 26. The method of claim 24further comprising positioning a parallel resistor across one of thesensing and compensating elements for providing the compensating elementwith lower resistance.